This invention relates to optical pick-ups used for recording and/or reading signals in optical disk systems.
A conventional optical pick-up for optical recording systems comprises a semiconductor laser, an optical lens for focusing the laser beam emitted from the semiconductor laser on to the optical disk, a beam splitter for splitting and guiding the reflected laser beam from the surface of the optical disk to photodetectors, a cylindrical lens for giving an astigmatism in the wavefront of the reflected beam to generate focusing error signals, and photodetectors for detecting focusing and tracking error signals and for detecting information signals stored in the optical disk.
Since the conventional optical pick-up for optical disk system comprises a plurality of small-sized optical components, the total size and weight are not sufficiently small to realize a thin and compact disk system. Also, the optical axes arrangements for all small-size optical components are very difficult, and thus the production cost becomes very expensive. Hence, optical pick-ups, in which some plurality of optical components are integrated into one monolithic or hydrid units, are proposed in order to realize a small-sized and light optical pickup system and to simplify the fabrication processes thereof. For example, an integrated thin film optical pick-up, which integrates the beam splitter photodetectors and lens on a silicon substrate was proposed by Ura et al. An holographic optical pick-up, in which the beam splitter and cylindrical lens are functionally integrated into a hologram has been developed by Kimura et al. And an optical pick-up with waveguide detectors in which beam splitters and photodetectors are integrated on the waveguide, fabricated on a transparent glass substrate, was proposed by Sagawa et al.
However, these proposed optical pick-ups have the following serious problems. The main problems in the integrated thin film optical pick-up are its low power utilization efficiency of the laser, and the poor focusing properties of the waveguide lens. Namely, the coupling efficiency between a laser diode and a waveguide which integrates a focusing lens and a beam splitter and photodetectors is less than 10%, and the coupling efficiency of the waveguide lens from guided mode to radiation optical mode should be kept less than 10%, and thus an optical laser power utilized to read information stored on a disk is limited. The focusing properties of the grating lens are very sensitive to the wavelength of the semiconductor laser, and the positional deviation from the design between the laser and the grating lens, and therefore focused spot size, is enlarged and several spots appear due to the wavelength shift or the multimode oscillation of the laser diode owing to the optical reflection. The big problem in the holographic pick-up, which is the second example, is the limitation of the size of the integrated device and the fine adjustment of the photodetectors and hologram with respect to the semiconductor laser in the integrated device. A sufficiently long distance, about 5 mm between the semiconductor laser and the hologram, is required because the photodetectors should be placed apart from the semiconductor laser about 1 mm. The photodetectors should be placed at an accurate position in order to obtain the focusing error and tracking error signals. The problem in the third example of the optical-up is the limited speed of the photodetectors. The photodetectors should be fabricated as an amorphous silicon, therefore the speed is limited due to the low electron mobility.